1. Field of the Invention
The present invention relates to an antenna module installing an antenna coil having a resonant frequency, and to an electronic apparatus installing the above-mentioned antenna module.
2. Description of the Related Art
Non-contact communication systems, which are called radio frequency identifications (RFIDs), are now in widespread use. As a non-contact communication method used in the RFID system, there can be exemplified a capacitive coupling method, an electromagnetic induction method, and a radio wave communication. Among them, the RFID system using the electromagnetic induction method is, for example, constituted of a first coil on a reader/writer side and a second coil on a transponder side. A data communication via the coils is performed by a magnetic coupling of the above-mentioned two coils.
Specifically, the reader/writer amplitude-modulates the magnetic field generated in the first coil so as to transmit data, and the transmitted data is detected on the transponder side. Further, the transponder performs a modulation process such as an amplitude modulation through a load switching (LS) of the second coil so as to transmit the data to the reader/writer.
The antenna coils of the transponder and the reader/writer each operate as an LC resonant circuit. In general, the resonant frequency of those coils is adjusted to be a carrier frequency of a carrier wave for communication and is resonated to set an appropriate communication distance between the transponder and the reader/writer.
Further, also non-contact power supply (non-contact power transmission, wireless power transmission) systems have received attention in recent years. As a power transmission method used in the non-contact power supply system, there can be exemplified an electromagnetic induction method and an electromagnetic resonance method. The electromagnetic induction method utilizes the same principle as previously described in the RFID system. Specifically, the magnetic field generated when current is supplied to the first coil is used to supply the electric power to the second coil. On the other hand, the electromagnetic resonance method includes an electric field coupling and a magnetic field coupling, and performs a power transmission by a coupling of the electric field or the magnetic field utilizing a resonance. In particular, the electromagnetic resonance method utilizing the magnetic field coupling has attracted attention in recent years, and the resonance antennae thereof are designed with coils.
In view of the above-mentioned principle, in the RFID system and the non-contact power supply system, a resonance design for the coil of the antenna portion (hereinafter, referred to as antenna coil) is very important. Even if the antenna coil itself is designed to resonate at a desired frequency, when the antenna coil is actually implemented in the electronic apparatus, magnetic field components generated from the antenna coil may be affected by metal located in the periphery of the antenna coil. Therefore, it is difficult for the antenna coil to exert the desired property. That is caused due to the fact that the magnetic field generated from the antenna coil interferes (is coupled to) the metal located in the periphery of the antenna coil with a result that inductance components of the coil is decreased and the resonant frequency is shifted and that eddy-current loss occurs.
As one of the countermeasures, a magnetic sheet has been utilized. When the magnetic sheet is provided between the antenna coil and the metal located in the periphery of the antenna coil, the magnetic fluxes generated from the antenna coil are collected in the magnetic sheet. Thus, the interference with respect to the metal is decreased. It should be noted that there is a limitation on thickness of the magnetic sheet for the tendency of downsizing or thinning the electronic apparatus and it is difficult to ensure a sufficient sheet thickness, and hence the magnetic sheet is incapable of completely eliminating the interference of the magnetic field with respect to the metal. Therefore, it is necessary to coordinate a resonance circuit design in order to achieve a desired property when implemented in the electronic apparatus, considering various combinations including the position at which the metal to be arranged in the electronic apparatus, the position of the antenna coil, the magnetic permeability property and the thickness of the magnetic sheet, and the like.
In the electronic apparatus, a battery is exemplified as one of the metal parts that exert the most effect on the antenna coil. When the battery, the antenna coil, and the magnetic sheet are implemented in the electronic apparatus as separate components, the coordination described above needs to be considered in the implementation phase. However, if the battery, the antenna coil, and the magnetic sheet are integrated as one component, positional relation between the battery, the antenna coil, and the magnetic sheet when implemented in the electronic apparatus can be grasped. Thus, even after the implementation, the resonant frequency of the antenna coil, which is adjusted before the implementation, is not largely changed, and hence it is unnecessary to substantially readjust the resonant frequency of the antenna coil during the implementation.
For example, Japanese Patent Application Laid-open No. 2007-124557 (Paragraph [0025], FIG. 3) (hereinafter, referred to as Patent Document 1) and Japanese Patent Application Laid-open No. 2007-165141 (Paragraph [0016], FIG. 5) (hereinafter, referred to as Patent Document 2) each have proposed a structure in which the antenna coil and the battery are integrated.
In Patent Document 1, a magnetic material having a frame shape or a band shape is arranged so as to surround the outer surface of the battery, and the antenna coil is wound on the outer surface of the magnetic material. Further, in Patent Document 2, a battery case is formed of a resin including magnetic filler, and the antenna coil is wound around the battery case.